Hauling capability

Hi RJR,

You're quite right that referencing pulling power in "cars" is a worthless assessment.  Unfortunately, the best data we have for pulling power estimates comes from magazine articles that sometimes reference some crudely derived (spring scale hooked to the coupler) drawbar measurements.  Such measurements are only given in the most thorough product reviews and are generally not included in the more casual "first impressions" type of article.  Manufacturers, as far as I know, have never published any kind of drawbar performance data.

Glad to have met you at York, Dave.  And pulling power does not equate to safe operating load. 

Safe operating load is certainly up for grabs.  I recently pulled 115 cars around with two Legacy U-Boats, including a 2% grade.  I'm pretty sure I was beyond the safe operating load for the locomotives!  You can certainly pull more than is recommended (whatever that number is) before your traction tires start slipping.

The determining factor in my book is the load on the system. If you don't already have one, I'd suggest an ammeter. I start paying close attention when continuous current starts running over 3A. It's sustained long term loads that get things heated-up.

Gilly

You find a static drawbar-pull number (X pounds @ Y volts) in many HO engine reviews but I can't recall seeing a published dynamic tractive-effort characterization (drawbar-pull x speed) for any engine in any scale.  

Every so often (usually in HO) an operating dynamometer car like the MOW prototypes comes up.  To date every one I've seen has been crudely implemented using the spring method (essentially a fish-scale) that Dave describes.  And I've never seen a dynamometer car that indicates not just drawbar-pull but actual power (scaled Horsepower) which accounts for speed and I think is what many people associate with load capability.

As I see it, there are too many variables to come up with a user-friendly number.  Your assessment of differences in car types is a good one.  Then, for example, smoke unit power goes through those same rectifiers and I can't imagine a manufacturer publishing that you lose 5 cars (or whatever) of tractive effort if you turn smoke on!  And practically speaking, I can only imagine the can-of-worms it opens up with MU configurations. If a manufacturer stated a particular engine pulls X cars, does that mean 2 engines can pull 2X cars?

I think Gilly may be on to the right approach, since amperage is what heats a motor.  Question now would be to determine a maximum safe continuous draw for each loco.  And of course, the ammeter reading will include lighting, smoke unit, and PS board power load,

An interesting calculation is to determine what is the end load on motor bearings, since that is where, ultimately, all the stress ends.  This involved calculating mechanical advantage (disadvantage) of the wheels (levers), spur gears, worm gear, all of which force the motor shaft forward or back.